Supersensitizers for optically sensitized photoconductive layers



United States Patent Ofiice 3,352,670 Patented Nov. 14, 1967 3,352,670SUPERSENSITIZERS FOR OPTICALLY SENSI- TEZED PHQTOCONDUflTIVE LAYERSRobert F. Coles, North St. Paul, Minn, assignor to Minnesota Mining andManufacturing Company, St. Paul,

Minn, a corporation of Delaware No Drawing. Filed Feb. 14, 1964, Ser.No. 344,811

7 Claims. (Cl. 96-15) -CRa= (N=CH),,

CHR3NH- where R is selected from the group consisting of hydrogen,alkyl, alkaryl and aryl; and n is selected from Oand 1.

This invention relates to the optical sensitization of photoconductivematerials and to methods of increasing the effectiveness of opticalsensitizing dyes in photoconductive compositions useful injmagerecording. In one aspect this'invention relates to additives which maybe incorporated into photoconductive zinc oxide systems with opticalsensitizers to produce a supersensitizing effect.

The optical sensitization of photoconductive materials has beendescribed in the literature. Photoconductive copysheets used inelectrolytic electrophotography, such as those described in US.Patent'Nos. 3,010,883 and 3,010,884, are desirably sensitized to improveand extend their spectral response and efficiency. In another imagerecording process, i.e., electrostatic printing, such as described inU.S. Patent Nos. 3,052,539 and 3,052,540, the photoconductive sheets orcoatings can also be dye sensitized. Because of the desirability ofobtaining a white sheet color, the concentration of sensitizing dye isnor- .mallymaintained at minimum levels consistent with im 'provedspectral response.

Although the concept of supersensitization has been known in silverhalide photography, it has not been possible merely to transfer suchinformation, and materials over to photoconductive copysheets.Supersensitization may be considered as the increase in sensitivitywhich is obtained by the addition of certain materials, not necessarilysensitizers per se, to a sensitized system and which is not the resultof a mere additive effect. In fact, supersensitization may be consideredto be an exception to the general rule that the admixture of sensitizingdyes tends to reduce the overall sensitivity below that level providedby the respective components used individually. Though it is believedthat supersensitization results from a mere efiicient transfer ofenergy, in the form of excited electrons, from the sensitizing dye tothe zinc oxide conduction band, the actual mechanism is still not fullyunderstood.

It is an object of this invention to provide supersensitizers for use inphotoconductive systems, particularly in photoconductive recordingelements.

Another object of this invention is to provide increased speed ofresponse in a photoconductive zinc oxide copysheet without objectionablesheet coloration.

Still another object of this invention is to provide photoconductivecopysheets suitable for use in electrolytic electrophotography havingincreased speed and sensitivity Without substantial increase in overalldark conductivity.

In accordance with this invention the above and other objects areachieved by incorporating into a dye sensitized photoconductive layer ofa photoconductive copysheet an organic compound which is a complexingagent for zinc ion and which contains the radical form a 5 or 6 memberedheterocyclic ring (e.g. morpholino, piperidino, piperazino, etc, and Yis where R is hydrogen, alkyl, alkaryl or aryl (including thesubstituted derivatives thereof) and n is 0 or 1.

Compounds such as N,N-dimethylaniline have essentially nosupersensitizing value when used in conjunction with inorganicphotoconductive materials (e.g., zinc oxide, cadmium sulfide, indiumoxide, etc), particularly as compared to the highly eifective ringsubstituted anilines such as p-dimethylaminobenzaldehyde andp-dimethylaminobenzoic acid. The s-upersensitizers of this invention maygenerally be used in amounts ranging from about 0.000l% to about 1.5% byweight of the photoconductive material, preferably from 0.0001% to about1.5% by weight. The preferred supersensitizers are not opticalsensitizers per se for photoconductive zinc oxide.

The qualitative determination of the relative. effectiveness of acompound as a supersensitizer can readily be accomplished by thefollowing method. A dispersion is prepared by ball milling for severalhours a mixture of 2867 grams of powdered zinc oxide (U.S.P. 12), 2821grams of toluene, milliliters of methanol and 2026 grams of a 30 weightpercent solution of styrene-butadiene copolymer (30/70 mol ratiorespectively) in toluene. The test compound is dissolved in a suitablesolvent, e.g. methanol, to make a 2 weight percent solution, and 3.1cubic centimeters of the resulting solution is added to 200 grams of thedispersion. After thorough mixing the sample is allowed to stand at roomconditions for about 24 hours. A 3.5 milliliter quantity of thefollowing sensitizing dye mixture is then added 'before coating:

54 milliliters of 0.2 weight percent Sul-fofiavine in methanol- 1 a 27milliliters of 0.2 weight percent Phloxine B (CI.

42410) in methanol 19 milliliters of 0.5 weight percent Alphazurine 2G(CI.

42045) in methanol Other optical sensitizing dyes (eig. cyanines,xanthenes, merocyanines, diand tri-phenyl methanes) can be used, sincethe supersensitizers of this invention have not been paper laminate, toprovide a dry coating thickness of about 0.7 mil. The coated substrateis then exposed with a standard light spectrogram source through acalibrated step wedge or a continuous grey wedge. and electroiyticallydeveloped in a known manner. A control sample in which the test compoundis omitted is prepared, exposed and electrolytically developed with thesame procedure and under the same conditions. Visual examination andcomparison of the developed samples will indicate whether the testcompound performs, a supersensitizin'g function in the photoconductivelayer.

Illustrative examples of such ring substituted analine compounds, all ofwhich exhibit at least a two-fold increase in sensitivity, are set forthin Table 1. Using the testing method given above, sensitivitymeasurements were made at 4450, 55 and 6500 angstroms. In general, themetaand para-substituted anilines, particularly nonionicpara-substituted anilines are preferred. When acids are used, thecorresponding salts, preferably the alkali or alkaline earth salts, mayalso be employed. The ability of the compound to form a complex withzinc ion can be determined in a conventional manner. Because of theenhanced sensitivity resulting from the use of such ring substitutedanalines, the amount of optical dye sensitizers included in thephotoconductive layer along with the organic binder and photoconductorpowder can be reduced, thereby improving sheet color. Essentially whiteor otf-white copysheets are most desired in electrolyticelectrophotography.

TABLE I ii cnzmmorr SOQN;

to about 1.5% by weight of photoconductive material of r asupersensitizer in said layer comprising a organic compound which is acomplexing agent for zinc ion and which contains the radical where R andR tare selected from the group consisting of hydrogen, alkyl, alkaryl,and atoms necessary to form a heterocyclic ring having from 5 to 6members; Y is a radical selected from the group consisting of where R isselected from the group consisting of hydrogen, alkyl, alkaryl and aryl;and n is selected from and 1.

2. In a light sensitive recording element having an optically sensitizedphotoconductive zinc oxide layer and being suitable for use inelectrolytic electrophotography, the improvement which comprises fromabout 0.0001% to about 1.5% by weight of zinc oxide of a supersensitizerin said layer comprising an organic compound which is a complexing agentfor zinc ion and which contains the radical Ra Y- where R and R areselected from the group consisting of hydrogen, alkyl, alkaryl, andatoms necessary to form a heterocyclic ring having from 5 to 6 members;Y is a radical selected from the group consisting of s II c.

-CR3= (N=CH)n- CHR3NH where R is selected from the group consisting ofhydro- 6 gen, alkyl, alkaryl and aryl; and n is selected from 0 and 1.3. The light sensitive recording element of claim 2 in which saidsupersensitizer is wherein R and R are as defined in claim 2. I

4. The light sensitive recording sheet of claim 2 in which saidsupersensitizer is Q /N R: SOaH or a salt thereof, where R and R are asdefined in claim 2.

References Cited UNITED STATES PATENTS 3,197,307 7/1965 Blake et a1961.8 3,265,497 8/1966 Kosche 96-1.5 3,271,143 9/1966 Sorensen 961.73,271,144 9/ 1966 Clausen et a1. 96-1 NORMAN G. TORCHIN, PrimaryExaminer.

C. E. VAN HORN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,352,670 November 14, 1967 Robert F. Coles It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below.

Column 2, line 43, for "0.001%" read 0.00015,

Signed and sealed this 3rd day of December 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

2. IN A LIGHT SENSITIVE RECORDING ELEMENT HAVING AN OPTICALLY SENSITIZEDPHOTOCONDUCTIVE ZINC OXIDE LAYER AND BEING SUITABLE FOR USE INELECTROLYTIC ELECTROPHOTOGRAPHY, THE IMPROVEMENT WHICH COMPRISES FROMABOUT 0.0001% TO ABOUT 1.5% BY WEIGHT OF ZINC OXIDE OF A SUPERSENSITIZERIN SAID LAYER COMPRISING AN ORGANIC COMPOUND WHICH IS A COMPLEXING AGENTFOR ZINC ION AND WHICH CONTAINS THE RADICAL